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The differences are really quite small, compared to the differences imposed by the wall being there in the first place.

Take a look at:
http://www.ko4bb.com/Manuals/05%29_G...2_and_5GHz.pdf

Admittedly that's comparing 2.4Ghz and 5.8Ghz - but it's a much bigger frequency difference and there's less public data available on the licensed bands.

You can see the difference in signal loss through most materials to be just 1-2dB between the two bands, with a few wild outliers due to resonances and so on, and a few cases where the higher frequency actually penetrates better.

As I've said before many times now, the difference between 2100, and 1800 are mostly down to the different technologies used and not the frequency.